1. Technical Field of the Invention
This invention relates mainly to a variable valve timing and lift device for automotive engines.
2. Prior Art
At present there are certain variable valve timing and lift devices for automotive engines in public knowledge, for example, those under the unexamined Japanese patents publications No. 55-500656, 61-250307, etc.
The devices mentioned in these publications consist of a multiple number of cams of different profiles and an equal number of rocker arms, and these are coupled to the intake valve or exhaust valve by a connecting mechanism. The connecting mechanism is switched from one cam to the other for varying the valve timing and lift according to the operational condition of the engine. Not only are these rocker arms and connecting mechanisms complicated in construction, their use for coupling the cams with the valves creates new problems because of the resulting drop in rigidity.
Said drop in rigidity is specially critical at high speed engine operation where high rigidity is required to meet high cam acceleration. Low rigidity way cause the valves to fail to keep pace with the cam profile, and the valves may jump and bounce. When this happens, the engine can no longer run at high speed. Thus the use of a variable valve timing and lift device for obtaining high output from low speed to high speed by making the valve timing and valve lift variable losses its meaning.
In order to overcome this difficulty, the unexamined Japanese patent publication No. 63-41611 has proposed an engine valve train where each valve is operated directly by a cam via a hydraulic lifter. In this construction, a low speed cam is fixed to the camshaft, and a high speed cam having cam profile larger than that of low speed cam is provided on the camshaft in such a way that it can move in the axial direction of the camshaft but cannot rotate relative to this shaft. A plunger that slides inside the camshaft under the action of hydraulic pressure in the oil passage provided in the camshaft and a return spring allow this high speed cam to come into contact and out of contact with the hydraulic lifter.
Since the rocker arms and connecting mechanisms stated earlier are not required in the construction mentioned in this publication, there is no danger of any chain reaction, such as a decrease in rigidity, leading to a problem for the valves to keep pace, and the consequent jumping and bouncing of the valves. However, since the hydraulic system provided for camshaft lubrication is also used here for moving the high speed cam, the hydraulic pressure originally meant for lubrication fluctuates, depending on whether the high speed cam is working or resting. This destabilizes the oil supply to the camshaft journal, posing the danger of seizure of the journal. Another problem here is that the camshaft construction becomes complicated, affecting rigidity, which in turn calls for enlarging the camshaft size to increase rigidity.
The unexamined Japanese patent publication No. 59-101515, on the other hand, proposes an internal combustion engine valve opening and closing device of the following construction. In this device where the cam fitted to the camshaft works to open and close the valve(s), a pair of pistons is provided in the mechanism that transmits force from the cam to the valve(s). In addition, tapered step or groove is provided in the piston that is nearer to the cam, and the two pistons are accommodated in an oil cylinder containing a relief hole to let out the oil.
The construction shown in this publication is simpler, and the fluctuation in the hydraulic pressure of the lubricating system is less, compared with the construction of the unexamined Japanese patent publication No. 63-41611. However, since the amount of lift is varied through the control of oil, which is always present between the pair of pistons, by relieving said oil as the tapered groove value (upper piston) turns while the valve is moved up and down by the cam, a lift loss occurs as the tapered groove (oil feed hole) opens and closes. The problem that results from this is that the actual valve lift in this case becomes smaller than the valve lift that the cam should normally produce.
On the other hand, the mechanism proposed in the unexamined Japanese patent publication No. 59-101515, employs in effect, the method of varying the amount of gap as the means for making the cam (valve) lift variable. Here, since a large gap is produced even at the time of maximum lift control, and also the lift is varied by rotating the piston (upper) and lengthening the relief time as stated above, the net amount of lift becomes small. Since the ramp on the cam profile that operates the valve mechanism in normal manner disappears at this time, the acceleration during the opening and closing of the valve becomes abnormally high, producing loud noise and occasionally bouncing of the valves. This is why it is very difficult to put the valve mechanism proposed in this publication into practical use in an internal combustion engine.